1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer using electrophotography and, more particularly, to an image forming apparatus having a control means for detecting an end of a recording medium by a photosensor, and correcting and controlling an image write position in a direction (main scanning direction) perpendicular to the convey direction of the recording medium on the basis of the detection result.
2. Related Background Art
FIG. 17 shows a method of determining an image write position in a direction (main scanning direction) perpendicular to the convey direction of a recording medium in a conventional image forming apparatus.
To record image information in a conventional image forming apparatus, the image forming apparatus comprises a convey guide 101 as a reference, as shown in FIG. 17. A recording medium 102 is conveyed while either a right or left end in the main scanning direction with respect to the recording medium convey direction abuts against the convey guide 101. For example, when the left side serves as the image write reference of the recording medium 102, the left end of the recording medium 102 abuts against the convey guide to convey the recording medium 102. With this arrangement, the recording medium 102 can always be conveyed at the same position, and image information can always be recorded at the same position on the recording medium 102 with respect to the end of the recording medium 102.
As a method of not moving a recording medium, the positional shift (deviation) amount of the recording medium in the main scanning direction is detected, and the image write timing is corrected. FIG. 18 shows a conventional image write timing correction method.
A sheet end detection means 103 is mechanically driven using a motor (not shown) or the like with respect to the recording medium 102. The sheet end detection means 103 passes the end of the recording medium 102, and then an output from the sheet end detection means 103 changes. The image forming apparatus calculates, e.g., a moving distance until the output changes after the start of moving the sheet end detection means 103 on the basis of the rotation amount of the motor. The image forming apparatus corrects the image write timing on the basis of the calculation result.
As a means for detecting the end of a recording medium without moving the sheet end detection means, a photosensor can be used. A conventional method of detecting the end of a recording medium by using a photosensor will be explained. FIG. 19 is a block diagram showing the image forming apparatus.
A controller 104 sends image data to a printer engine 105 on the basis of a horizontal sync signal (BD signal) and vertical sync signal (TOP signal) sent from the printer engine 105. For example, the controller 104 sends image data to the printer engine 105 on the basis of the horizontal sync signal (BD signal) upon reception of the vertical sync signal (TOP signal) sent from the printer engine 105.
FIG. 20 is a schematic view showing the printer engine 105. The printer engine 105 includes a sheet end detection means 106, a registration roller 107, an attraction roller 108 which makes an electrostatic convey belt 113 attract a recording medium, image forming portions 109 to 112, the electrostatic convey belt 113, a feed unit 114, a fixing device 115, a fixing/discharge portion 116, and double-side convey portion 117.
The outline of an image forming operation will be described by exemplifying double-side recording, with reference to FIGS. 19 and 20.
An image is formed on the second surface (lower surface) of a recording medium. The recording medium is conveyed from the feed unit 114, and the registration roller 107 removes any skew. The sheet end detection means 106 between the registration roller 107 and the attraction roller 108 detects the sheet end (e.g., left end) of the recording medium in the main scanning direction. Then, the recording medium is attracted by the electrostatic convey belt 113 by using the attraction roller 108, and conveyed to the image forming portions 109 to 112.
The sheet end detection means 106 uses a photosensor to detect a recording medium in a noncontact state.
The printer engine 105 notifies the controller 104 of positional information about the detected end of the recording medium as status information. The controller 104 controls the image data transmission timing to the printer engine 105 on the basis of that received positional information. The controller 104 adjusts the image write position in the main scanning direction. An image is formed on the recording medium at the image forming portions 109 to 112.
The image formed on the recording medium is fixed as a permanent image onto the recording medium by heat and pressure at the fixing device 115. The recording medium is discharged by a predetermined amount from the fixing/discharge portion 116, and then the fixing/discharge portion 116 is reversely rotated to convey the recording medium to the double-side convey portions 117.
The recording medium reaches the registration roller 107 again through the double-side convey portions 117, and the registration roller 107 removes any skew. The sheet end detection means 106 detects the sheet end (e.g., left end) of the recording medium in the main scanning direction again, and an image is formed on the first surface (front surface) by the same method as that for the image on the second surface (back surface).
A method of detecting the end of a recording medium will be described. FIGS. 21 and 22 show the sheet end detection means. The sheet end detection means is made up of an LED 135, a transparent material 136 which guides light from the LED 135, and a CIS (Contact Image Sensor) 137 which detects light from the LED 135 as an analog signal.
FIG. 23 is a control block diagram showing the sheet end detection means, and FIG. 24 is a timing chart of the sheet end detection means. The printer engine 105 comprises the sheet end detection means 106, a binarization means 118, an oscillation unit (X′ tal) 119, an ASIC 120, a CPU 121, a BD detection means 122 which receives a BD signal serving as the reference of an image write position in the main scanning direction, and a TOP generation means 123 which generates a TOP signal serving as the reference of an image write position in the subscanning direction.
A TOP signal serving as the reference of an image write position in the subscanning direction is generated (T1801). At a predetermined timing, a detection start signal Start Pulse is input from the ASIC 120 to the sheet end detection means 106 (T1802). Start Pulse is used as a reset pulse for a CIS 34 and initializes the CIS 34. The LED 135 is turned on by an LED ON/OFF signal from the ASIC 120 (T1803).
The LED 135 emits light under predetermined driving conditions, and the light from the LED 135 irradiates the CIS 34 via the transparent material 136. The CIS output changes to “H” at a portion irradiated with light, whereas the CIS output changes to “L” at a portion where light is shielded by a recording medium. After that, a detection clock SNS CLK is input from the ASIC 120 to the sheet end detection means 106 (T1804).
The sheet end detection means 106 outputs the detection results of the CIS 34 as analog signals sequentially from the end in synchronism with the clock pulse SNS CLK. An output (analog output) from the sheet end detection means 106 is binarized by the binarization means 118 and input to the ASIC 120 (T1805).
The ASIC 120 generates a WINDOW signal which enables an output from the binarization means 118 (T1806). During the effective (H) period of the WINDOW signal, the ASIC 120 counts the number of H outputs from the binarization means 118 in synchronism with the clock pulse SNS CLK (T1807).
The CPU 121 calculates the distance of a light permeable portion in the sheet end detection means 106 from the count result, and detects the end of the recording medium.
The controller 104 is notified of positional information about the end of the recording medium as status information.
In the conventional image forming apparatus, a recording medium is mechanically moved to a normal position during the conveyance of the recording medium, as shown in FIG. 17. When recording media subjected to recording processing have a variety of sizes, it is difficult to move recording medium of all sizes to the normal position.
To mechanically move a recording medium during conveyance, the recording medium must be released from the convey means. This is difficult to realize when the recording medium is conveyed vertically from a lower portion to an upper portion or from an upper portion to a lower portion.
It is therefore difficult to always maintain the same image write position with respect to the recording medium at low cost and high precision.
Particularly when an image on the second surface is recorded in double-side recording, the convey path of a recording medium is unstably long and the image write position is more difficult to match.
In the method of moving the sheet end detection means to detect the positional shift amount of a recording medium and correcting the image write position, the detection means must be mechanically driven to a position where the means can detect at least either the right or left end of the recording medium with respect to the convey direction. The apparatus requires a bulky arrangement and cannot be downsized.
Further, a motor for driving the detection means is needed, resulting in an expensive system.
In the method of detecting the end position of a recording medium and correcting the image write position in accordance with the detected end position of the recording medium, the end position of the recording medium may be erroneously detected. In this case, the image write position is controlled based on the erroneously detected result.
Especially when a photosensor is used as a means for detecting the end position of a recording medium, the light emission portion of the photosensor keeps driving conditions constant. For example, the light amount of the light emission side decreases, failing to obtain a predetermined sensor output due to a decrease in the light amount of the light emission portion in accordance with the service life, or dust at the light emission portion under the influence of toner scattering or the like caused by long-term use in the image recording apparatus.
Alternatively, the light receiving amount decreases due to dust at the light receiving portion, and a predetermined sensor output cannot be attained.
As a result, image information cannot be recorded at a normal position on a recording medium.